Motion related artifacts, which interfere with the ECG signal presented to the ECG monitor, are common when using standard diagnostic type ECG equipment in a strong magnetic field environment, such as that presented in the tunnel of an MRI machine. FIG. 3A shows a representative normal ECG signal, while FIG. 3B shows a representative ECG signal including an error signal "e" (i.e., the voltage artifact) represented by the difference between the dotted line and the solid line therein.
One type of motion artifact is created by induction in the connecting leads or directly from the patient through the ECG electrodes. One example of this type of motion artifact is caused by patient breathing. The electrode leads, which are attached to the electrodes on the patient's chest, move in the strong magnetic field. Voltages are induced in the leads as a result of lead motion and are combined with the cardiac voltage at the input to the monitor. Another example of this type of motion artifact is one caused by patient blood flow. Since the blood is an electrical conductor that is moving in a magnetic field, rapid blood flow in the aorta and other large heart vessels generates interfering signals. This artifact combines with the cardiac voltage within the patient and is picked up on the skin by the ECG electrodes.
One method of correcting for motion artifacts in a ECG measurement made in the magnetic field of an MRI machine is described in an article in the January, 1987 issue of Investigative Radiology, entitled "Optimizing Electrocardiograph electrode Placement for Cardiac-Gated Magnetic Resonance Imaging" by Dimick et al. In this method, the ECG electrodes are placed in predetermined positions on the patient to minimize the motion artifact. The authors concede that this method is not totally effective in suppressing the blood flow artifact. Also, the detection of certain heart abnormalities requires that the ECG electrodes be placed in particular positions which may be inconsistent with the positions necessary for eliminating the motion artifact.
A second method for correcting for motion artifacts is described by a Request for Proposal by the Department of Health and Human Services, Research Grant entitled "ECG Monitoring in MRI to Detect Cardiac Ischemia" (date unknown). The Request for Proposal indicates that the National Heart, Lung, and Blood Institute is seeking "a device that can detect myocardial ischemia from the ECG (in the MRI environment) despite the voltages induced by blood flow artifacts." The Request for Proposal suggests that by "monitoring the ECG signals from an array of electrodes placed on the chest both before and after the patient is placed in the magnet . . . [t]he difference between these two sets of signals is due to the voltages induced by the rapid blood flow through a magnetic field [and] [d]igital signal processing might then be used to reconstruct and display the original ECG." This method can eliminate the motion artifacts so long as the patient's ECG signal remains static. However, if there is a change in the ECG signal during an MRI scan, the signal processor subtracting the motion artifact may interpret this as a change in the motion artifact, since a dynamic change in the patient's ECG signal and a change in the motion artifact are indistinguishable except through the manipulation of data previously collected. This can result in inaccurate ECG measurements being output.
It is therefore an objective of the present invention to provide a method and system for reducing motion artifacts in ECG measurements made in strong magnetic fields.
It is yet another objective of the present invention to provide a system for reducing motion artifacts in ECG measurements made in strong magnetic fields in which the placement of the ECG electrodes is not limited to certain positions on the patient.
It is a further objective of the present invention to provide a system for reducing motion artifacts in ECG measurements made in strong magnetic fields which operates in real time and is not dependent on prior measurements.